Weldability of aluminum alloys is often defined by their susceptibility to the formation of hot tears since this weld defect is the most prevalent one found and depends primarily upon composition of the alloy; whereas, other defects are more process-and-procedure-related. The hot tear defect manifests itself in the form of a crack generated behind the solidification front of a weld and therefore is deleterious in terms of weld metal mechanical integrity. Hot tears will form during the welding process if the alloy possesses an inherent susceptibility and sufficient strain exists in the weld to cause their formation. The major factors which contribute to the alloy's susceptibility to this defect are the coarseness of the solidification structure, the amount and species of the alloy elements and the geometry of the welded joint (restraint stresses). The crystallographic nature of solidification ordains that solidifying grains of weld metal will assume the orientation of their parent seed crystals which are adjacent to the fusion zone. Even though competitive growth between grains will exist owing to the relationship between preferred growth directions and the thermal gradients in the weld pool, the nucleation of new grains with more favorable orientations is seldom observed in the absence of heterogeneous nuclei. A low rate of nucleation of new weld metal grains allows the growth of large columnar grains, leading to segregation of elements at the centerline which may enhance hot tear formation in a localized region. Therefore one of the keys to controlling hot tearing susceptibility is to refine the weld metal structure.
In aluminum alloys certain alloying elements show a higher propensity to the hot tearing phenomena. The solutions employed in the past in the area of alloying have been to avoid the use of certain alloys with high susceptibilities, or to use filler wire alloy compositions which reduced the susceptibility as a result of dilution effects which change weld bead chemistry during welding.
Prior art improvements have been achieved in the weldability of aluminum alloys through refinement of the weld metal grain size by adding an element which will provide some type of substrate upon which new grains of weld metal can form. The two most common elements used in aluminum alloys to achieve this heterogeneous nucleation have been titanium and zirconium. These two elements produce refinement because an intermetallic compound is formed when they are combined with aluminum, existing in the stoichiometric ratio of 1:3 for titanium or zirconium to aluminum (i.e., TiAl.sub.3 and ZrAl.sub.3). Particles of this intermetallic compound provide a substrate upon which new grains of weld metal may form through a peritectic reaction mechanism, thereby causing a refinement in the weld metal structure.